Circuit for reducing pulse pile-up in pulse direction systems by converting a random pulse train to that of fixed frequency

ABSTRACT

The amplified pulses from a radioactivity detector are coupled to a discriminator and via a 400 nanosecond delay line to the inputs of three charge and hold circuits. The discriminator drives a first modulo 3 scaler-decoder and also a single shot multivibrator. The single shot output and the decoder outputs are AND gated with a sense amplifier sensing the charge and hold capacitor to control the charge and hold circuits. A second modulo 3 scaler-decoder circuit driven by a clock provides the gating of three interrogator circuits connected to the outputs of the three charge and hold circuits, respectively. The circuit converts the random pulse train from the radioactivity detector to that of a train of pulses of a fixed frequency.

Utlitfid States Patent [1 1 Culver l CIRCUIT FOR REDUCING PULSE PILE-UP IN PULSE DIRECTION SYSTEMS BY CONVERTING A RANDOM PULSE TRAIN TO THAT OF FIXED FREQUENCY [75] Inventor: Richard B. Culver, Houston, Tex.

[73] Assignee: Dresser Industries, Inc., Dallas, Tex.

[22] Filed: June 1, 1971 [21] Appl. No.: 148,754

DETECTOR I I MODULE) 3 DISCRIMINATOR DECODER [11]] 3,745,346 July 10, 1973 Primary Examiner-James W. Lawrence Assistant ExaminerD. C. Nelms:

Attorney-Robert W. Mayer, Thomas P. Hubbard, Jr., Daniel Rubin, Raymond T. Majesko, Roy L. Van Winkle, William E. Johnson, Jr., Eddie E. Scott and Morgan L. Crow [57] ABSTRACT The amplified pulses from a radioactivity detector are coupled to a discriminator and via a 400 nanosecond delay line to the inputs of three charge and hold circuits. The discriminator drives a first modulo 3 scalerdecoder and also a single shot multivibrator. The single shot output and the decoder outputs are AND gated with a sense amplifier sensing the charge and hold capacitor to control the charge and hold circuits. A sec ond modulo 3 sealer-decoder circuit driven by a clock provides the gating of three interrogator circuits connected to the outputs of the three: charge and hold circuits, respectively. The circuit converts the random pulse train from the radioactivity detector to that of a train of pulses of a fixed frequency.

6 Claims, 1 Drawing Figure NEUTRON SOURCE LINE g MULTIVIBRATQR AND 64 6| ,-GATE I CLOCKI-L MODULOB SCALER DECODER i I {om LINE AMPLIFIER SINGLE SHOT MULTIVIBRATOR AND GATE 66 3 A 5 AMP CHARGE AND HOLD r1 3 1st \NTERROGATOR |v SINGLE SHOT MULTIVIBRATOR lNTERROGATOR Patented July 10, 1973 I0 DETECTOR J MODULO 3 f 60 DISCRIMINATOR SCALER 5 DECODER 73 ll 3 ,0 2 S CLOCK MODULO 3 SCALER I DECODER NEUTRON T 0 33 SOURCE LINE DELAY AMPLIFIER LINE M f 1 sINGLE SHOT 74 MULTIVIBRATOR I A SINGLE SHOT MULTI VIBRATOR I 6? 70 AND 64 GATE 6| INTERROGATOR 'vw CHARGE AND 75 HOLD I sINGLE SHOT MULTIVIBRATOR ,68 65 7 62 INTERRDGATDR 'VV 4| a CHARGE AND HOLD 1: SINGLE SHOT MULTIVIBRATOR S69 72 66 2 3 INTERROGATOR "-'VV\I'- LGI-IARGE AND INVENTOR HOLD RICHARD B. CULVER mm PW ATTORNEY CIRCUIT FOR REDUCING PULSE PILE-UP IN PULSE DIRECTION SYSTEMS BY CONVERTING A RANDOM PULSE TRAIN TO THAT OF FIXED FREQUENCY RELATED APPLICATIONS This application relates to my co-pending US. Pat. application, Ser. No. 146,261, filed May 24, 1971, entitled CIRCUIT FOR IMPROVING DATA IN PULSE DETECTION SYSTEMS, and to my concurrently filed co-pending US. Pat. application Ser. No. 148,765, filed June 1, 1971, entitled CIRCUIT FOR REDUC- ING PULSE FILE-UP IN PULSE DETECTION SYS- TEMS.

BACKGROUND OF THE INVENTION This invention relates generally to the art of geophysical prospecting and more particularly to the art of radioactivity well logging involving the counting of pulses resulting from the detection of such radioactivity.

For neutron activation analysis and for some thermal neutron capture measurements that are made with a pulsed neutron source, the counting rates are quite high and many pulses are lost due to the dead time of the surface analysis system. Also, pulses may pile-up on the line and cause the spectrum to be distorted.

Sodium iodide detectors are normally used in subsurface spectral measurement systems because they offer the best compromise of physical and electrical characteristics. Good linearity and resolution can be obtained with detector output pulse widths of about 1.2 microseconds. However, when the pulses vare transmitted over a well logging cable, any pulse arriving at the surface for analysis will have been spread in time to about ten microseconds by its transit over the four miles of cable that typically separates the surface and subsurface systems. The fast multichannel analyzers that are presently available have pulse pair resolving times of about 15 microseconds for pulses of this width.

It is therefore the primary object of the present invention to provide a circuit which substantially reduces the problem of pulse pile-up on the line; and It is still another object of the invention to provide a new and improved circuit for converting a random pulse train from the detector to a fixed frequency pulse train.

SUMMARY OF THE INVENTION The objects of the invention are accomplished,

broadly, by a circuit which utilizes a combination of energy discrimination and delay logic for eliminating pulse pile-up by delaying the processing of any pulses that would otherwise follow too soon after any given pulse, wherein means are provided for converting the random pulse train from a detector to a fixed frequency pulse train.

These and other objects, features and advantages of the invention will become apparent to those skilled in the art from the following detailed description, when considered with the accompanying drawing, in which the sole FIGURE is a block circuit diagram of the invention.

Referring to the drawing in more detail, there is illus trated in block diagram a circuit which can be used to aid in reducing the probability of pulse pile-up within instrumentation which is used in conducting a radioactivity well survey. It should be appreciated that the circuits embodied herein find utility in my abovereferenced co-pending applications and the disclosure of such application is incorporated herein by reference.

The detector 10, which may be used in a well logging instrument not illustrated, is a scintillation counter arranged for gamma ray spectral analysis in accordance with prior art. In the preferred embodiment, it is a Na] (Tl) crystal having a two inch diameter with a three inch length, such crystal being coupled to a Model Number 4518 photomultiplier tube available from the Radio Corporation of America. Such a crystal-photomultiplier combination may have a resolution of 7.5 to 8.5 percent with a 660 kev cesium 137 peak. The output of the detector 10 is coupled into a discriminator l l, which in turn is coupled into one of the three inputs to the AND gate 12. The discriminator l l is used to remove pulses having amplitude below a selectable threshold value which might otherwise cause pile-up on the transmission line normally connected to the output terminal 33. A setting of 400 kev is generally suitable for this purpose. The output of the discriminator 1] drives a first modulo 3 sealer-decoder circuit 60 and also triggers a single shot multivibrator circuit 14. The clock 13 drives a second modulo 3 sealer-decoder circuit 73. Also, the clock 13 may be appropriately scaled by a scaler (not illustrated) to synchronize the source with the detection system. The scaled clock pulse activates a high voltage pulser (not illustrated) arranged to cause the neutron output of neutron source 15 to pulse at an appropriate multiple or submultiple of the clock frequency.

The amplified pulses from the detector 10 are also coupled into the delay line 16, which is nominally set to cause a delay of 0.4 microseconds, the output of which is coupled into the three charge and hold circuits 40, 41 and 42.

The output of the single shot multivibrator 14 is coupled into the first gate on each of the three input AND gates 61, 62 and 63, respectively, the outputs of which are coupled into the charge and hold circuits 40, 41 and 42, respectively. p

The 0 outputof the scaler-d ecoder circuit 60 is connected to the second input of the three input AND gate. The 1 output of the decoder circuit 60 is connected to the second input of the three input AND gate 62 and the number 2" output of the decoder circuit 60 is connectedto the second input of the three input AND gate 63. A trio of sense amplifiers 64, 65 and 66 are connected to the capacitors C1, C2 and C3, respectively, for sensing whether such capacitors are charged. The outputs of the amplifiers 64, 65 and 66 are connected'to the third inputs of the AND gates 61, 62 and 63, respectively. Interrogator circuits 67, 68 and 69 are connected, respectively, to the capacitors C1, C2 and C3. The capacitor C1 is associated with the charge and hold circuit 40 for the storage of any detector pulse appearing in the circuit 40. The interrogator circuit 67, for example a linear gate circuit, also has means for discharging the capacitor C1 in a manner known in the art since such interrogator circuits are conventional. The outputs of the interrogators 67, 68 and 69 are coupled, respectively, through the resistors 70, 71 and 72 to the line amplifier 32.

The clock 13 also drives a second modulo 3 sealerdecoder circuit 73, the outputs of which are connected to drive three single shot multivibrators 74, 75 and 76,

respectively, which in turn gate the interrogator circuits 67, 68 and 69, respectively.

In the operation of the circuit, the detector pulses I from detector are coupled tothe discriminator 11 puts fromthe decoder circuit 60 and the output from one of the sense amplifiers 64, 65 and 66 determine if the particular charge and hold circuit is already storing a pulse and together cause oneof the AND gate circuits 61, 62 and 63 to control its respective charge and hold circuit 40, 41 and 42. The second modulo 3 sealerdecoder circuit 73 is driven by the clock 13 with a frequency slightly lower than the pulse pair resolving frequency of any limiting element in the analysis system. The outputs of the decoder circuit 73 drive the single shot multivibrators 74, 75 and 76, respectively, to control the interrogator circuit of the particular channel. The interrogator outputs are fed to the line amplifier 32 and output terminal 33 for transmission to the earths surface,and the output terminal 33. The output termihal 33 correspondsto a point which would normally be associated with the amplification and transmittal of pulses to the earths surface in well logging utility. Thus, it should be appreciated that pulses are sequentially stored at random time intervals in the charge and hold circuits but are read out at fixed time intervals as determined by the clock frequency, the sealer-decoder circuit 73'and the single shot multivibrator-interrogator combinations.

Again it should be appreciated that thediscriminator '11 maybe used to reject unwanted low energy pulses 1 longer than the pulse pair resolving time of the limiting element in the system. The maximum number of pulses per second that 'can be processed by a system is the reciprocalof the resolving-time and any pulse that might "occur during the resolvingtime interval immediately following a pulse being processed is lost.

""Sinceradioactive' decay is distributed in a Poisson distribution, the probability that one or more random pulses occur in a given time interval'immediately fol 'lowing another pulse is Where n number of pulses in the interval N mean pulse rate I measuring interval.

Therefore, for N N maximum,

The probability that no pulses are lost is The average number of pulses lost during each resolving time interval during which a pulse is being processed is Therefore, when Nt l, 50 percent of the pulses in a random pulse train will be lost. Thus, if the circuit in the sole FIGURE stores a maximum of three pulses, having three channels, the data loss can be reduced from about 50 percent to about 15 percent when the mean pulse rate equals the reciprocal of the resolving time of a system. By adding a fourth channel, the loss can be reduced to about 5 percent.

For ease of illustration, the circuit has been described for processing a maximum of three detector pulses per gating interval. It should be appreciated, however, that more or less pulses can be processed by a different number of channels. The length of the gating interval and the decay time of the detector will determine the maximum. This method has been shown to greatly increase the data rate when fast detectors are used, such as with solid state or other fast detectors.

If it is desirable to process all detector pulses, the discriminator is set just above the noise level. However, additional pulse selection can be made by adjusting the threshold to eliminate undesirable low energy pulses.

, For sake of clarification, it should be appreciated that the sense amplifiers 64, and 66 are ,inhibit type amplifiers wherein a signal is applied to the third input of the AND gates 61, 62 and 63 only when there is no charge on the respective capacitors C1, C2 and C3.

sive property or privilege is claimed are defined as follows: j v

1. In an electrical circuit in which electrical pulses are cyclically and randomly associated with a synchronized event, the improvement comprising:

clock means for producing a seriesof synchronizing gate pulses; an input terminal for receiving electricalpulses associated with said synchronizing gate pulses; delay means connected to said input terminal; a plurality of charge and hold circuits connected to i the output of said delay line; I i means to sequentially gate said charge and hold circuits in response to signals appearing at said input terminal; and v means connected to said clock means to interrogate said charge and hold circuits at a fixed frequency subsequent to each such synchronized gate pulse. 2. ln an.electrical circuit in which electrical pulses are cyclically and randomly associated with a synchronized event, the improvement comprising:

clock means for producing a series of synchronizing gate pulses; an input terminal for receiving electrical pulses associated with said synchronizing gate pulses; I delay means connected to said input terminal;

N number of charge and hold circuits connected to the output of said delay line;

The embodiments of the invention in which an exclumeans to sequentially gate said charge and hold circuits, including a first modulo N sealer-decoder connected to said input terminal, where N is any number more than one, in response to signals appearing at said input terminal; and means connected to said clock means to interrogate said charge and hold circuits at a fixed frequency subsequent to each such synchronized gate pulse. 3. In the circuit according to claim 6, wherein said means to interrogate includes a second modulo N scaler-decoder connected to gate N number of interrogator circuits connected to the outputs of said N number of charge and hold circuits, respectively.

4. In the circuit according to claim 3, being further characterized as including discriminator means between said input terminal and the input of said first can be gated only if its said respective capacitor is not charged. 

1. In an electrical circuit in which electrical pulses are cyclically and randomly associated with a synchronized event, the improvement comprising: clock means for producing a series of synchronizing gate pulses; an input terminal for receiving electrical pulses associated with said synchronizing gate pulses; delay means connected to said input terminal; a plurality of charge and hold circuits connected to the output of said delay line; means to sequentially gate said charge and hold circuits in response to signals appearing at said input terminal; and means connected to said clock means to interrogate said charge and hold circuits at a fixed frequency subsequent to each such synchronized gate pulse.
 2. In an electrical circuit in which electrical pulses are cyclically and randomly associated with a synchronized event, the improvement comprising: clock means for producing a series of synchronizing gate pulses; an input terminal for receiving electrical pulses associated with said synchronizing gate pulses; delay means connected to said input terminal; N number of charge and hold circuits connected to the output of said delay line; means to sequentially gate said charge and hold circuits, including a first modulo N scaler-decoder connected to said input terminal, where N is any number more than one, in response to signals appearing at said input terminal; and means connected to said clock means to interrogate said charge and hold circuits at a fixed frequency subsequent to each such synchronized gate pulse.
 3. In the circuit according to claim 6, wherein said means to interrogate includes a second modulo N scaler-decoder connected to gate N number of interrogator circuits connected to the outputs of said N number of charge and hold circuits, respectively.
 4. In the circuit according to claim 3, being further characterized as including discriminator means between said input terminal and the input of said first scaler-decoder.
 5. In the circuit according to claim 3, being further characterized as including discriminator means between said input terminal and said sequential gating means.
 6. In the circuit according to claim 3, wherein said gating means includes N number of AND gates, said N number of charge and hold circuits include N number of capacitors, respectively, and being further characterized as including N number of inhibit sense amplifiers connected between said capacitors and one input of said AND gates, respectively, whereby said AND gates can be gated only if its said respective capacitor is not charged. 